Applicants claim priority of Japanese application, Ser. No. JP 11-024209, filed Feb. 1, 1999.
This invention relates to a carburetor for a two stroke engine and more particularly to a carburetor which provides to the engine a scavenging air supply in addition to a fuel and air mixture supplied to the engine.
It has been proposed in two stroke engines to provide a scavenging air supply to facilitate removal of exhaust gases from the engine cylinder and to provide a buffer between the incoming fresh fuel and air mixture and the outgoing exhaust gases to inhibit the escape through the exhaust port of the fresh fuel and air mixture before its combustion. While effective for its intended purpose, the introduction of scavenging air, in addition to the air within the fresh fuel and air mixture delivered to the engine, provides a resulting fuel and air mixture which is too lean to support abrupt or rapid acceleration of the engine which requires a somewhat rich fuel and air mixture. Sometimes, when rapidly accelerated, the engine may stall and cease to operate because the scavenging air makes the fuel and air mixture too lean for such acceleration.
In prior carburetors having a fuel and air mixing passage and a scavenging air passage, an air valve in the scavenging passage has been mechanically linked in some manner to a throttle valve in the fuel and air mixing passage. In these carburetors, the extent to which the air valve is opened depends on the extent to which the throttle valve is opened. Therefore, when the throttle valve is rapidly opened, such as to provide an abrupt acceleration of the engine, the air valve also opens providing additional scavenging air into the engine which can lead to a fuel and air mixture which is too lean to support the abrupt acceleration of the engine. Therefore, a need exists to control the air valve independently of the throttle valve at least during rapid acceleration of the engine so that a relatively rich fuel and air mixture may be supplied to accelerate the engine.
A carburetor for a stratified type scavenging engine has a scavenging air supply passage with an air valve therein, a fuel and air mixing passage having a throttle valve therein, a diaphragm which defines a negative pressure chamber on one side and an atmospheric chamber on its other side, and a low pressure passage that communicates a venturi portion of the fuel and air mixing passage with the negative pressure chamber. The air valve is responsive to movement of the diaphragm caused by the pressure differential across it to move the air valve between open and closed positions and a spring yieldably biases the air valve towards its closed position. The air valve is not directly mechanically connected or linked to the throttle valve. Thus, when the throttle valve is rapidly opened during engine acceleration, the air valve is not immediately rapidly opened and is not significantly opened until a sufficient pressure differential exists across the diaphragm. Therefore, during initial rapid acceleration of the engine from idle, the air valve prevents a significant flow of scavenging air to the engine to ensure a sufficiently rich fuel and air mixture is supplied to the engine to support the rapid acceleration.